An image display device called a head-up display (HUD) displays information which is required for operation and control in a cockpit of an aircraft or an automobile (e.g. speed information or altitude information). The automobile driver or aircraft pilot may perceive the information displayed by the HUD like the information displayed in front of the windshield.
An image display device called a head-mounted display (HMD) is worn like ordinary eyeglasses for vision correction. A user wearing the HMD may perceive images displayed by the HMD like the image situated in a space in front of lens portions.
Both of the HUD and the HMD allows a user to perceive the image through substantially transparent members such as a windshield or lens portions. Therefore, those image display devices are called “see-through display devices”. Such image display devices have been well developed in recent years.
For example, a driver of an automobile on which an HUD is mounted may visibly recognize necessary information for a drive under slight movements of the line of vision while the driver looks forward during the drive. Therefore, the HUD ensures high safety and convenience.
The HMD may provide a user with a large image at a very low level of power consumption. The user may view images at any location and obtain necessary information anywhere and anytime.
The see-through display has to mix external light (natural light) incident from the outside such as a perspective with images to be displayed. For example, an HUD for an automobile uses a combiner nearby the windshield to mix images to be displayed with external light incident from the outside. It is preferred to decrease optical loss in the external light incident from the outside and desired images to be displayed while the images to be displayed are mixed with the external light incident from the outside.
A conventional see-through display device uses a volume hologram as a combiner (c.f. Patent Document 1). If a hologram is used as a combiner, the image displayed by the HUD spreads as a result of lens effect of the hologram. Consequently, a user may view a large image even when the see-through display device is small.
Volume holograms have high diffraction efficiency specifically only for a predetermined wavelength. For example, if a laser source is used as a light source and if a volume hologram is designed so as to have high diffraction efficiency for a wavelength range corresponding to wavelength of a laser beam from the laser source, the HUD may achieve high light utilization efficiency with decreased loss of the natural light.
A volume hologram is exposed in order to form interference fringes in the volume hologram used for the HUD. During the exposure process for the volume hologram, interference fringes are also formed by reflected light from an interface of the volume hologram. It is known that the interference fringes formed by the reflected light at the interface of the volume hologram cause stray light.
Generation principles of stray light in a conventional HUD are described with reference to FIGS. 32 to 35. FIG. 32 is a schematic view of the HUD into which a conventional reflective volume hologram is incorporated. FIG. 33A is a schematic view of an exposure optical system of the HUD shown in FIG. 32. FIG. 33B is a schematic view showing a positional relationship among a main light beam in the exposure optical system shown in FIG. 33A, the volume hologram and the observer. FIGS. 34A and 35 are schematic views of an optical path of stray light in the HUD shown in FIG. 32.
The conventional HUD is described with reference to FIG. 32.
The conventional HUD 900 includes a laser source 910, which emits a laser beam LB, and a projection optical system 920, which generates image light IL from the laser beam LB. The projection optical system 920 includes a lens 921, which receives the laser beam LB from the laser source 910, a return mirror 922, which changes a propagation direction of the laser beam LB from the lens 921, a liquid crystal panel 923, which receives the laser beam LB from the return mirror 922 and generates the image light IL, a projection lens 924, which receives the image light IL from the liquid crystal panel 923, and a screen, 925 which receives the image light IL projected by the projection lens 924.
The HUD 900 further includes a controller 930. The controller 930 controls the laser source 910 and the liquid crystal panel 923 to generate the image light IL for displaying desired images.
For example, the HUD 900 is mounted on a vehicle. FIG. 32 shows a windshield 940 of the vehicle. The windshield 940 is used as a part of the HUD 900. The windshield 940 includes an inner glass 941 defining a space (interior space), in which a driver D exists, and an outer glass 942 forming a boundary with a space (exterior space) outside the vehicle. The driver D is an observer observing images displayed by the HUD 900.
The HUD 900 further includes a volume hologram 950 situated between the inner and outer glasses 941, 942. The volume hologram 950 deflects the image light IL projected from the projection optical system 920 toward the driver D.
The laser source 910 of the HUD 900 emits the laser beam LB. The lens 921 of the projection optical system 920 spreads the laser beam LB. The spread laser beam LB is returned by the return mirror 922 toward the liquid crystal panel 923. Consequently, the laser beam LB enters the liquid crystal panel 923.
The liquid crystal panel 923 forms a desired pattern two-dimensionally under the control performed by the controller 930. The laser beam LB passing through the liquid crystal panel 923 is spatially modulated and becomes the image light IL. The image light IL is projected on the screen 925 from the projection lens 924.
The image light IL emitted from the screen 925 is incident on the volume hologram 950 sandwiched between the inner and outer glasses 941, 942. The volume hologram 950 diffracts the incident image light IL toward the driver D. Consequently, the driver D may view a virtual image VI of the image projected on the screen 925 through the windshield 940.
Paths of the image light IL emitted from the screen 925 and the external light (sunlight, light from tail lamps of vehicles in front of the host vehicle, and headlights of vehicles behind the host vehicle) from the outside of the vehicle are sufficiently taken into account in a design of the HUD 900 shown in FIG. 32. However, as a result of diffraction by the volume hologram 950, unintended light in the design may enter a view of field of the driver D (observer) because of diffraction by the volume hologram 950. The unintended light is referred to as “stray light” hereinafter.
FIG. 33A is a schematic view of the exposure optical system of the volume hologram 950. The optical system for recording interference fringes in the volume hologram 950 of the HUD 900 is described with reference to FIGS. 32 and 33A. The volume hologram 950 functions as a reflective hologram.
The exposure optical system 960 includes a half mirror 961 configured to receive a laser beam RLB which has the same wavelength as the laser beam LB emitted by the laser source 910 described with reference to FIG. 32. The half mirror 961 divides the laser beam RLB into an object light OL and a reference light RL.
The exposure optical system 960 further includes a lens 962, which receives the object light OL, and a pinhole plate 963 situated between the lens 962 and the volume hologram 950. A small hole is formed in the pinhole plate 963.
The object light OL is directed from the half mirror 961 to the lens 962. The lens 962 concentrates the light on the small hole of the pinhole plate 963. Consequently, the object light OL passing through the pinhole plate 963 becomes spherical waves. The object light OL then enters the volume hologram 950.
The exposure optical system 960 further includes a return mirror 964, which deflects the reference light RL toward the volume hologram 950, a lens 965, which receives the reference light RL from the return mirror 964, and a pinhole plate 966 situated between the lens 965 and the volume hologram 950. A small hole is formed in the pinhole plate 966 for the object light OL, like the pinhole plate 963.
The reference light RL propagates from the half mirror 961 toward the return mirror 964. The return mirror 964 returns the reference light RL toward the lens 965. The lens 965 concentrates the light on the small hole of the pinhole plate 966. Consequently, the reference light RL passing through the pinhole plate 966 becomes spherical waves.
The volume hologram 950 includes a surface 951, which the object light OL enters, and a surface 952 opposite to the surface 951. The reference light RL is incident on the surface 952.
The pinhole plate 963 is positioned and angularly set with respect to the volume hologram 950 so that a position of the small hole of the pinhole plate 963, through which the object light OL passes, corresponds to a central region of the screen 925 of the HUD 900 described with reference to FIG. 32. In FIG. 32, the distance from the volume hologram 950 to the central region of the screen 925 is shown by the symbol “L2”. Likewise, as shown in FIG. 33A, the distance from the small hole of the pinhole plate 963 to the volume hologram 950 is “L2”.
The pinhole plate 966 is positioned and angularly set with respect to the volume hologram 950 so that a position of the small hole of the pinhole plate 966, through which the reference light RL passes, corresponds to a central region of the virtual image VI created by the HUD 900 described with reference to FIG. 32. In FIG. 32, the distance from the volume hologram 950 to the central region of the virtual image VI is shown by the symbol “L1”. Likewise, as shown in FIG. 33A, the distance from the small hole of the pinhole plate 966 to the volume hologram 950 is “L1”.
If the volume hologram 950 is irradiated for a predetermined time with the object and reference lights OL, RL under the aforementioned optical settings of the exposure optical system 960, interference fringes are recorded in the volume hologram 950. Accordingly, the volume hologram 950 functions as a transmissive hologram as described above.
FIG. 33B schematically shows a path of the main beam in the exposure optical system 960. FIG. 33B shows an optical system after the pinhole plates 963, 966. Only the main beams OMB, RMB of the object and reference lights OL, RL are shown in FIG. 33B to make the generation principles of stray light easily understood.
The generation principles of stray light are described by using the main beams OMB, RMB. However, the same generation principles of stray light are applicable not only to interference between the main beams OMB, RMB but also to other interference generated by two light fluxes.
If light is incident on a transparent object, which is different from the surrounding space (air) in a refractive index, the light is partially subjected to Fresnel reflection at the boundary between the surrounding space and the transparent object.
In FIG. 33B, a space forming the boundary with the surface 951 of the volume hologram 950 is called “interior space”. A space forming the boundary with the surface 952 of the volume hologram 950 is called “exterior space”.
The main beam OMB of the object light OL enters the surface 951 of the volume hologram 950, and then reaches the surface 952. The main beam OMB is partially subjected to Fresnel reflection according to the aforementioned principle. Consequently, the reflected light OMR of the main beam OMB is generated.
The main beam RMB of the reference light RL enters the surface 952 of the volume hologram 950, and then reaches the surface 951. The main beam RMB is partially subjected to Fresnel reflection according to the aforementioned principle. Consequently, the reflected light RMR of the main beam RMB is generated.
As a result of the aforementioned Fresnel reflection, four light beams pass through the volume hologram 950. Consequently, interference fringes generated by interference among the four beams are recorded in the volume hologram 950.
In the following description, the interference fringes formed by the interference between the main beams OMB, RMB of the object and reference lights OL, RL is called “interference fringe 1”. The interference fringe formed by the interference between the main beam OMB of the object light OL and the reflected light OMR of the main beam OMB is called “interference fringe 2”. The interference fringe formed by the interference between the main beam RMB of the reference light RL and the reflected light RMR of the main beam RMB is called “interference fringe 3”. The interference fringe formed by the interference between the reflected lights OMR, RMS of the main beams OMB, RMB of the object and reference lights OL, RL is called “interference fringe 4”. The interference fringe formed by the interference between the main beam OMB of the object light OL and the reflected light RMR of the main beam RMB of the reference light RL is called “interference fringe 5”. The interference fringe formed by the interference between the main beam RMB of the reference light RL and the reflected light OMR of the main beam OMB of the object light OL is called “interference fringe 6”.
As described above, the six interference fringes are formed on the volume hologram 950. A modulation amount of refractive indexes of the interference fringes 2 to 6 is less than that of the interference fringe 1.
Three interference fringes among the six interference fringes formed in the volume hologram 950 cause stray light directed toward the driver D. The interference fringes causing the stray light are the “interference fringe 1”, “interference fringe 3” and “interference fringe 6”.
FIG. 34A shows schematically the stray light caused by the interference fringe 1. FIG. 34B shows schematically the stray light caused by the interference fringe 3. FIG. 35 schematically shows the stray light caused by the interference fringe 6. The volume hologram 950 shown in FIGS. 34A to 35 is incorporated in the HUD 900. Therefore, FIGS. 34A to 35 show the volume hologram 950 sandwiched between the inner and outer glasses 941, 942. FIGS. 34A to 35 schematically show the optical system after the pinhole plates 963, 964 to clarify the generation principles of the stray light although the object light OL and reference light RL does not exist after the volume hologram 950 is incorporated in the HUD 900.
FIG. 34A schematically shows the generation principles of the stray light caused by the interference fringe 1. The stray light caused by the interference fringe 1 is described with reference to FIG. 34A.
External light enters the outer glass 942. FIG. 34A shows an external light component EC1 incident on the outer glass 942 at an incidence angle as great as the incidence angle of the main beam OMB of the object light OL on the inner glass 941. The external light component EC1 sequentially passes through the outer glass 942, volume hologram 950 and inner glass 941, and then reaches the boundary between the inner glass 941 and the interior space. The external light component EC1 is partially subjected to Fresnel reflection at the boundary between the inner glass 941 and the interior space, and then propagates again toward the volume hologram 950. The external light component EC1 is then partially diffracted by the interference fringe 1 recorded in the volume hologram 950. Accordingly, the external light component EC1 is partially emitted in the same direction as the main beam RMB of the reference light RL. Thus, the external light component EC1 is partially perceived as the stray light by the driver D.
FIG. 34B shows schematically the generation principles of the stray light caused by the interference fringe 3. The stray light caused by the interference fringe 3 is described with reference to FIG. 34B.
There is also external light incident on the inner glass 941 from the exterior space. FIG. 34B shows an external light component EC2 emitted at an angle as great as the emission angle of the reflected light RMR of the main beam RMB of the reference light RL emitted from the outer glass 942. The external light component EC2 is incident on the inner glass 941 from the interior space. The external light component EC2 then passes through the inner glass 941, and then reaches the volume hologram 950. The external light component EC2 is diffracted by the interference fringe 3 recorded in the volume hologram 950 and emitted in the same direction as the main beam RMB of the reference light RL. Thus, the external light component EC2 is perceived as the stray light by the driver D.
FIG. 35 shows schematically the generation principles of the stray light caused by the interference fringe 6. The stray light caused by the interference fringe 6 is described with reference to FIG. 35.
The interference fringe 6 is formed to allow optical transmission inside the volume hologram 950. FIG. 35 shows an external light component EC3 incident at the same incidence angle as the reflected light OMR of the main beam OMB of the object light OL. The external light component EC3 is diffracted by the interference fringe 6, propagates in the same direction as the main beam RMB of the reference light RL, and is emitted from the inner glass 941. Thus, the external light component EC3 is perceived as the stray light by the driver D.
If interference fringes are recorded by two-light flux interference in the volume hologram 950, as described above, interference is caused by the Fresnel reflected light generated at an interface between the volume hologram 950 and the air. As a result of the interference exposure by the Fresnel reflected light, unintentional interference fringes are recorded in the volume hologram 950. Therefore, stray light directed toward the driver D (observer) is generated.
Patent Document 2 suggests suppressing generation of the Fresnel reflected light at an interface of the volume hologram by means of optical contact liquid dropped between the volume hologram and the non-reflective plate. Since the Fresnel reflected light is less likely to occur, there is little stray light.
If the non-reflective plate and the optical contact liquid are used like the disclosed techniques in Patent Document 2, in order to cause little stray light, process steps increases before exposure process, in which the volume hologram is exposed. In addition to the non-reflective plate and optical close-contact liquid, dedicated equipment is required to use these.
Patent Document 1: JP 2007-526498 A
Patent Document 2: JP 2001-331084 A